164 Leaves and their Work 



But the fact is not so ; for the supply is equally 

 distributed. More fires are kept burning, and more 

 carbon-dioxide is produced in winter, when the trees 

 are leafless and do not want it, than in the summer, 

 when they do. And yet we are not choked by it, or 

 even inconvenienced by it, in the winter months, so it 

 must be got rid of somehow. For, if there were two 

 per cent, in the air, we should have severe headache, 

 and ten per cent would suffocate us. What, then, 

 becomes of it ? Roughly speaking, we may say that 

 the carbon-dioxide produced during the winter of the 

 north goes to feed the vegetation of the south — the 

 thistles, clover and grass, for instance, of the Pampas, 

 which are flourishing in all their luxuriance while 

 winter prevails with us. And it goes, to some extent, 

 at least, because the leaves of the southern hemisphere 

 draw it thither. 



The ocean of air which surrounds the world is not, 

 it must be remembered, a compound, but a mixture. 

 If we could see it we should find oxygen, nitrogen, 

 carbon-dioxide, ammonia, all perfectly mixed, but 

 perfectly distinct. The combination of two gases, 

 oxygen and hydrogen, makes water — a liquid entirely 

 different from both ; but there is no such combination 

 and alteration in the gases of the air. Each keeps its 

 own character ; but, though all are of different weights, 

 they are so thoroughly and perfectly mixed that, except 

 under special circumstances, there is but little appreci- 

 able difl"erence in the air of different parts of the world. 



Carbon-dioxide is the heaviest of these gases, and it 

 is more than twice the weight of the mixture of these 

 gases which we call the air. Where it is poured out from 

 cracks in the earth, as it is largely in some volcanic 



